A modular multilevel converter (MMC) comprises a plurality of cell converters (chopper circuits) connected in cascade. Each cell converter includes a capacitor and switching elements for controlling the voltage between output terminals to be a zero voltage or a capacitor voltage (see, for example, Japanese Patent No. 5378274: PTD 1).
The MMC, which has a plurality of cell converters connected in cascade, can output a voltage that is greater than or equal to the withstand voltage of the switching elements constituting each cell converter. Therefore, the MMC is expected to be applied to a high-voltage direct current (HVDC) system and a static synchronous compensator (STATCOM).
PTD 2 (WO 2014/148100) discloses a means for protecting cell converters from a DC short-circuit circulating current if a DC short-circuit accident occurs due to a lightning strike onto a DC overhead transmission line in an HVDC system.
Specifically, each cell converter includes: a main circuit constituted of switching elements and a DC capacitor; an external terminal for connection to another cell converter in cascade; and freewheel diodes each connected in anti-parallel with a corresponding one of the switching elements. A cell block is formed for each of a plurality of cell converters. A bypass circuit is connected to two external connection terminals of each cell block. If a DC short-circuit accident occurs, the bypass circuit passes a DC short-circuit circulating current, instead of the freewheel diodes in each cell block. Each of the bypass circuits that has an enough current capacity relative to the DC short-circuit circulating current would be able to protect each cell block from the DC short-circuit circulating current.